alt Hacker NewsWhy “obviously”? The file could contain two parts, one encrypted PowerPoint part with all data and original assets, and the latter just the “rendered” version with everything pruned down to what is needed to display the content. Pair this with some hashing which is signed/stuffed in the encrypted part and any PowerPoint copy would be able to detect tampering as well. Seems obviously trivial.
It's non-trivial to determine "what is needed to display the content". The closest you have is something like "print/save to pdf", which is too lossy for some (eg. you lose animations and comments), and isn't 100% foolproof either (eg. if you use rectangle tool to redact some text, you can still recover what's underneath).